LKML Archive on
help / color / mirror / Atom feed
From: Simon Arlott <>
To: Eric Dumazet <>
Cc:, Bill Irwin <>,
	Linux Kernel Mailing List <>,
Subject: Re: [PATCH (update 3)] timer: Run calc_load halfway through each round_jiffies second
Date: Fri, 02 Mar 2007 17:32:51 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 02/03/07 16:35, Eric Dumazet wrote:
> On Friday 02 March 2007 16:15, Simon Arlott wrote:
>> Whenever jiffies is started at a multiple of 5*HZ or wraps, calc_load is
>> run exactly on the second which is when tasks using round_jiffies will
>> be scheduled to run. This has a bad effect on the load average, making
>> it tend towards 1.00 if a task happens to run every time the load is
>> being calculated.
>> This changes calc_load so that it updates load half a second after any
>> tasks scheduled using round_jiffies.
> I believe this patch is too complex/hazardous and may break exp decay 
> computation.

Only for a single calculation whenever it has to adjust, which should only 
happen every 49.7 days (on 32-bit archs). (Or 5 minutes after booting... 
I always wondered why that happened and now I see it's initialised so it 
always wraps early). Whilst it is in sync with jiffies it will not affect 
the process - count is just set to the current value every time. Even with 
NO_HZ because jiffies will be correct when calc_load is called.

> (Even if nobody care about avenrun[] those days :), do you ? )
> You could just change LOAD_FREQ from (5*HZ) to (5*HZ+1)
> You can see that 5.01 instead of 5.00 second gives the same EXP_xx values.
> So (5*HZ + 1) is safe. (because HZ >= 100)

On HZ=1000, this would cause the load average to be pushed towards +1.00 
for up to 2 minutes every ~83 minutes with no obvious cause. (If a task 
takes ~10-20ms to run, so 20 runs are needed at HZ=1000 before it passes 
it again).

On HZ=100 it would happen every ~8 minutes for up to 10 seconds and never 
be noticed.

Using 5*HZ+2 would move this to ~167 and ~17 minutes which would mitigate 
the effect further still without changing the exp values.

1884.25 -> 1883.62
2014.15 -> 2014.02
2036.65 -> 2036.61

Will anyone notice if the load is adjusted slightly less frequently?

If this is considered preferable to adjusting calc_load to avoid almost all 
round_jiffies scheduled tasks (some of which may take longer than ~15ms to 
run), then I have no problems with it - I just needed something to stop my 
driver changes doing odd things to the load average for other people. I'll 
continue to run with this version, is it possible to add a Kconfig option 
for it somewhere?

Simon Arlott

  reply	other threads:[~2007-03-02 17:33 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-24 15:19 round_jiffies and load average Simon Arlott
2007-03-01  9:11 ` Simon Arlott
2007-03-01 18:52   ` [PATCH] timer: Add an initial 0.5s delay to calc_load Simon Arlott
2007-03-01 22:52     ` [PATCH (updated)] timer: Run calc_load halfway through each round_jiffies second Simon Arlott
2002-01-01  3:05       ` Pavel Machek
2007-03-05 22:35         ` Simon Arlott
2007-03-06 18:42           ` [PATCH (update 4)] " Simon Arlott
2007-03-06 22:20         ` [PATCH (updated)] " Chuck Ebbert
2007-03-01 23:10       ` Bill Irwin
2007-03-02 10:15         ` [PATCH (update 2)] " Simon Arlott
2007-03-02 15:15           ` [PATCH (update 3)] " Simon Arlott
2007-03-02 16:35             ` Eric Dumazet
2007-03-02 17:32               ` Simon Arlott [this message]
2007-03-02 18:03                 ` Eric Dumazet
2007-03-02 20:14                   ` Simon Arlott
2007-03-02 22:32                     ` Eric Dumazet
2007-03-02 23:54                       ` Simon Arlott

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \
    --subject='Re: [PATCH (update 3)] timer: Run calc_load halfway through each round_jiffies second' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).